Repeated evolution of drag reduction at the air-water interface in diving ANGOR UNIVERSITY kingfishers Crandell, K. E.; Howe, R. O.; Falkingham, P.L. Journal of the Royal Society: Interface DOI: 10.1098/rsif.2019.0125 PRIFYSGOL BANGOR / B Published: 31/05/2019 Peer reviewed version Cyswllt i'r cyhoeddiad / Link to publication Dyfyniad o'r fersiwn a gyhoeddwyd / Citation for published version (APA): Crandell, K. E., Howe, R. O., & Falkingham, P. L. (2019). Repeated evolution of drag reduction at the air-water interface in diving kingfishers. Journal of the Royal Society: Interface, 16, [20190125]. https://doi.org/10.1098/rsif.2019.0125 Hawliau Cyffredinol / General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. 01. Oct. 2021 Under review for J. R. Soc. Interface Repeated evolution of drag reduction at the air-water interface in diving kingfishers Journal: Journal of the Royal Society Interface Manuscript ID rsif-2019-0125.R2 Article Type:ForResearch Review Only Date Submitted by the n/a Author: Complete List of Authors: Crandell, Kristen; Bangor University, Biology Howe, Rowan; Bangor University, School of Natural Sciences Falkingham, Peter; Liverpool John Moores University, School of Natural Sciences and Psychology Categories: Life Sciences - Earth Science interface Biomechanics < CROSS-DISCIPLINARY SCIENCES, Evolution < CROSS- Subject: DISCIPLINARY SCIENCES Keywords: plunge diving, bird hydrodynamics, beak, bow wave, Alcedinidae http://mc.manuscriptcentral.com/jrsi Page 1 of 31 Under review for J. R. Soc. Interface 1 2 3 1 4 5 6 2 Repeated evolution of drag reduction at the air-water interface 7 8 3 in diving kingfishers 9 10 4 11 12 13 5 Crandell, KE*, Howe, RO*, Falkingham, PL ** 14 15 6 16 17 7 * School of Natural Sciences, Bangor University 18 19 For Review Only 20 8 ** School of Natural Sciences, Liverpool John Moores University 21 22 9 23 24 10 ABSTRACT 25 26 27 11 Piscivorous birds have a unique suite of adaptations to forage under the water. One 28 29 12 method aerial birds use to catch fish is the plunge dive, wherein birds dive from a 30 31 13 height to overcome drag and buoyancy in the water. The kingfishers are a well- 32 33 34 14 known clade that contains both terrestrially foraging and plunge-diving species, 35 36 15 allowing us to test for morphological and performance differences between foraging 37 38 16 guilds in an evolutionary context. Diving species have narrower bills in the dorso- 39 40 41 17 ventral and sagittal plane and longer bills (size corrected data, n=71 species, p<0.01 42 43 18 for all), Although these differences are confounded by phylogeny (phylogenetically 44 45 46 19 corrected ANOVA for dorso-ventral p=0.26 and length p=0.14), beak width in the 47 48 20 sagittal plane remains statistically different (p<0.001). We examined the effects of 49 50 21 beak morphology on plunge performance by physically simulating dives with 3D 51 52 53 22 printed models of beaks coupled with an accelerometer, and through computational 54 55 23 fluid dynamics (CFD). From physically simulated dives of bill models, diving species 56 57 58 59 1 60 http://mc.manuscriptcentral.com/jrsi Under review for J. R. Soc. Interface Page 2 of 31 1 2 3 24 have lower peak decelerations, and thus, enter the water more quickly, than 4 5 6 25 terrestrial and mixed-foraging species (ANOVA p=0.002), and this result remains 7 8 26 unaffected by phylogeny (phylogenetically corrected ANOVA p=0.05). CFD analyses 9 10 27 confirm these trends in three representative species, and indicate that the 11 12 13 28 morphology between the beak and head is a key site for reducing drag in aquatic 14 15 29 species. 16 17 30 18 19 For Review Only 20 31 Keywords: plunge diving, avian hydrodynamics, beak, bow wave, Alcedinidae 21 22 32 23 24 33 25 26 27 34 INTRODUCTION 28 29 35 30 31 36 Plunge diving has evolved in multiple flying species to facilitate transitioning 32 33 34 37 between the air and water – two mediums of vastly different densities. Birds 35 36 38 including gannets, terns, and boobies have mastered diving from air into water to 37 38 39 access fish meters below the surface. Morphological adaptations likely compliment 39 40 41 40 this foraging strategy in order to both improve dive efficiency and avoid damage on 42 43 41 water entry. The shape of the kingfisher’s bill has served as inspiration as a drag- 44 45 46 42 reducing structure for the Japanese Shinkansen Bullet train (1, 2). However, these 47 48 43 functions have yet to be directly tested. 49 50 44 The conversion of gravitational potential energy to kinetic energy during the 51 52 53 45 dive provides momentum for the bird to overcome body drag and buoyancy in order 54 55 46 to dive deeper (3). Birds are particularly buoyant due to the layer of air trapped 56 57 58 59 2 60 http://mc.manuscriptcentral.com/jrsi Page 3 of 31 Under review for J. R. Soc. Interface 1 2 3 47 between the body and the feathers, typically used for insulation (4), as well as body 4 5 6 48 fat and the avian system of airsacs (5). In the diving species the Lesser Scaup 7 8 49 (presumably already adapted to reduce drag), over 80% of work during a dive is to 9 10 50 overcome the significant costs of body buoyancy (6). 11 12 13 51 Minimizing the energetic costs of drag have led to streamlined bauplans in 14 15 52 swimming and flying animals (7-11). Bird beaks appear well-adapted to avoid both 16 17 53 aerodynamic and hydrodynamic drag. Most beaks are relatively cone-shaped, with a 18 19 For Review Only 20 54 small initial surface area relative to the direction of oncoming flow – thus reducing 21 22 55 immediate profile drag. The gradual increase in cross-sectional area allows flow to 23 24 56 remain laminar as it travels toward the wide middle-section of the animal. 25 26 27 57 While much work has focused on how shape influences drag across flying 28 29 58 and swimming animals, less work exists examining morphological function at the 30 31 59 air-water interface. Diving involves the animal rapidly transitioning between two 32 33 34 60 fluids of different physical properties – from air, a relatively low density and 35 36 61 viscosity fluid, to water, a higher density and viscosity fluid. Due to the high speed of 37 38 62 entry, diving comes at the cost of an initial impact at the water’s surface. Gannets 39 40 41 63 reportedly dive from a height of 30 meters in the air– a fall resulting in a speed of 22 42 43 64 m/s when impacting the water (3). While these impact speeds could seriously 44 45 46 65 damage a human entering feet-first (12), an avian injury due to water entry has not 47 48 66 been reported. The neck musculature coupled with streamlined beak and skull help 49 50 67 the gannet avoid injury by reducing impact forces (12). In fact, large decelerations 51 52 53 68 due to water impact during diving may not occur in birds. Accelerometers mounted 54 55 69 to free-living Cape Gannets sampling at 16 to 32 Hz detected no or minimal 56 57 58 59 3 60 http://mc.manuscriptcentral.com/jrsi Under review for J. R. Soc. Interface Page 4 of 31 1 2 3 70 deceleration due to impact during foraging dives. (3). Drag reduction due to 4 5 6 71 morphology may help reduce immediate impact forces. The hydrodynamic shape of 7 8 72 the avian bill may also reduce turbulence during the initial dive, which may help 9 10 73 avoid visual or vibrational detection by the prey (13). 11 12 13 74 Recent work examining water piercing by geometric cones suggests that 14 15 75 beak morphology may be selected on to reduce impact force, and thus, drag on entry 16 17 76 (14). The lower the opening angle of the cone (or the tip angle), the lower impact 18 19 For Review Only 20 77 forces and more smooth the transition between air and water (14). The opening 21 22 78 angle of a cone (a) can be calculated as a= 2*arcsin(r/s), where r is the radius of the 23 24 79 base, and s is the length of the side from base to tip (also called ‘slant height’). Thus, 25 26 27 80 to decrease the angle of a cone, either the radius of the base (r) must decrease, or 28 29 81 the length (s) must increase. If diving species of kingfisher are morphologically 30 31 82 adapted to minimize drag, we would expect them to have longer bills with a 32 33 34 83 narrower base relative to terrestrial species.